This invention relates to a drum brake equipped with a device that automatically adjusts the shoe clearances, and more specifically relates to a drum brake clearance adjustment staying device that halts this clearance adjustment when the brake temperature rises.
This type of device has been disclosed in the publication of Examined Japanese Patent Application Number 2(1990)-11768. A drum brake is generally fitted with an automatic shoe clearance adjustment device. This is a device in which a protrusion at one end of a bell crank lever, mounted to pivot on a strut, interlocks in a hole provided in one shoe web, with a certain amount of slack in the direction of movement of the brake shoe. When wear of the brake shoe lining would cause the amount of expansion of the brake shoes to exceed the slack, the bell crank lever pivots to expand the diameter between the two brake shoes, thereby automatically reducing the excess clearance.
The brake clearance adjustment device functions in the same manner should the brake drum thermally expand with friction. Hence, when the brake drum cools and contracts, the clearance between it and the brake shoe becomes much too narrow. A brake clearance adjustment staying device was developed to resolve this problem, and comprises a flat temperature sensor, which starts to deform at a specified temperature, with its base end riveted to the shoe web and a detector component, bent and molded at the free end of the temperature sensor, interlocking in the hole in which the protrusion of the bell crank lever is inserted. If the brake temperature starts to rise, the detector component of the temperature sensor slips out of the hole in the shoe web and starts to deform in the direction to actually increase the above mentioned slack.
Conventional clearance adjustment staying devices with such a temperature sensor are subject to certain difficulties.
The temperature sensor must be pre-mounted to a segment of the brake shoe with rivets. Accordingly, handling and stacking during shipping of a brake shoe with a mounted temperature sensor could deform the component. If the brake shoe has to be replaced because of lining wear, the entire unit including the temperature sensor has to be replaced. The temperature sensor is affixed to the shoe web by rivets inserted into round holes bored through both materials, after which the rivets are caulked; however, caulking of the rivets is tedious, requiring special caulking equipment. Since the shoe web and the temperature sensor are fitted together with rivets inserted in round holes, the temperature sensor can rotate freely relative to the shoe web, whereby the detector component of the temperature sensor can slip out of the hole in the web, and even become stranded on top of the web, where it will lose its basic function.